1. Field of the Invention
The present invention relates to optical apparatuses such as a television lens and a video lens, and, more particularly to an optical apparatus having an automatic focus function.
2. Description of the Related Art
Conventionally, various automatic focus (AF) techniques in imaging apparatuses such as a camera and a video camera have been proposed. For example, light beams from an object that have passed different exit pupil regions of a photographing lens are imaged on a pair of line sensors and a relative position displacement amount of a pair of image signals obtained by photoelectrically converting object images is calculated. An automatic focusing method for calculating a defocus amount of the object on the basis of the displacement amount and performing driving of the photographing lens is well known.
In moving image photographing, it is necessary to control the position of a focus lens to automatically perform focusing constantly.
Japanese Patent Application Laid-Open No. H04-217237 discloses a method of predicting, when it is temporarily impossible to perform focus detection, a moving position of an object based on the moving speed of the object in the past and maintaining a focus state.
Japanese Patent Application Laid-Open No. H08-101337 discloses a method of adaptively switching lens driving according to a focus detection result in order to obtain a focus state.
However, in the related arts disclosed in Japanese Patent Application Laid-Open No. H04-217237 and Japanese Patent Application Laid-Open No. H08-101337, it is impossible to constantly maintain the focus state in various photographing environments during taking moving images. It is difficult to instantaneously perform focus adjustment on different objects when panning or the like.
A focus detection unit images a light beam from an object on a line sensor and photoelectrically converts an object image. Therefore, time required for accumulation of charges (hereinafter referred to as charge accumulation) of the sensor is different depending on condition such as brightness of the object.
FIG. 9 shows a change in distance between a photographing lens and an object moving toward the photographing lens (moving upwardly in FIG. 9) as the time changes t1 to t7 and a change in distance in focus of the photographing lens (distance between the photographing lens and an object in focus) with respect to the time t1 to t7. In the figure, the ordinate represents a distance and the abscissa represents time. In the figure, a focal length of a focus lens at the time of focus on the object changes from P1 to P3 with respect to time t1 to time t7. At time t1, the focus lens is in a state of the focal length P1, the sensor is activated, and the charge accumulation is started. At time t2, the charge accumulation of the sensor is completed.
When the charge accumulation of the sensor is completed, data based on charges accumulated in the sensor is read and a defocus amount is calculated. At time t3, a defocus lens driving signal 801 is output to set a focal length of the focus lens to P2.
The focus lens traces a locus illustrated in the figure according to the driving signal 801, fits within depth of field, and can maintain the focus state. From time t3, the focus lens is driven. The focus lens stops when the focus lens is driven to obtain the focal length P2 as a target position.
However, when the contrast of the object falls, at time t3, the focus lens is stopped in a state of the focal length P2. When the charge accumulation takes time until time t6, update of a control signal for driving the focus lens is kept waited until time t7. Therefore, at time t7 when a signal 802 for driving the focus lens is output, the focus lens substantially deviates from the depth of field and cannot maintain the focus state. In such a phenomenon, as moving speed of the object is higher, time when the focus lens deviates from the depth of field is earlier. Therefore, it is more difficult to maintain the focus state.
As an object distance is shorter or the focal length of the lens is larger, the depth of field is smaller and it is more difficult to maintain the focus state.